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Will it just not boot up all the way but just stops?
If so try the procedure at this link: T-mobile Dash Master Reset
If this doesn’t work then proceed to the following guide below.
The Windows Mobile installation may be corrupted.
The only way to fix that problem is by re-installing the windows software on the phone.

A big black mark (often looks like an ink blotch) on a mostly white screen usually means that the screen was somehow damaged/broken. The Dash is a rather good device; however, it's been known to experience cracked (inner) screen issues on a slightly more frequent basis than other comparable mobile devices.

The company that I work for sees this issue a few times a month with the Dash, and unfortunately it's just a matter of replacing it at this point.

I'm sorry this isn't a happier answer.
Please let us know if you have any other questions!
Thanks,
Jonathan

1. PRESS AND HOLD BOTH SOFT KEYS.2. WHILE HOLDING DOWN SOFT KEYS, PRESS AND RELEASE THE POWER BUTTON, WAIT 3-5 SEC. (PHONE SHOULD POWER ITSELF ON BY DOING THIS)3. RELEASE THE TWO KEYS SOFT KEYS.4. WHEN SCREEN ON DASH LOOKS LIKE IT'S IN DOS FORMAT, PRESS SEND (GREEN ANSWER KEY)

- If this does not work you can try to flash the phone back to factory. Look up TMOBILE DASH factory flash.

do a sw reset....its possible it was damaged when it was dropped in the water, but not likey....

turn the phone off, press and hold the soft keys (buttons with the lines on them above the home key and back arrow key) and hold it down.....press the power button while you are doing that until the white t-mobile screen will come on, then instead of going to the green windows mobile screen, a black screen with white writing......press the green button....it will get you in your phone but it will erase your info as a result....

do a sw reset. turn the phone off, press and hold the soft keys (buttons with the lines on them above the home key and back arrow key) and hold it down.....press the power button while you are doing that until the white t-mobile screen will come on, then instead of going to the green windows mobile screen, a black screen with white writing......press the green button....it will get you in your phone but it will erase your info as a result....